I don't think it matters as long as you are always making sure it will be compatible on smaller and larger browser sizes. I personally use a 21.5 iMac 4K. But if I were given the choice to have a 27" I would take it, because the bigger the screen, the easier it is to size down the browser and see what a MBP sees rather than have a small screen and then have to look on how it looks on bigger screens by going on to independent computers.
But one benefit I have found with a MBP is that I can travel and still be doing Webflow while my iMac sits at home.
I think the ideal layout will be to include a MBP, and then a larger Apple Display for home which you can hook up the MBP to for a better screen size and display.
Hope this helps.